The present invention relates to a high-modulus glass composition. More particularly, this invention relates to a glass composition suitable for use as a substrate for information recording media, which is required to have excellent surface smoothness and a high modulus. The present invention further relates to a substrate for information recording media, which comprises the glass composition, and to an information recording medium.
Information recording devices such as magnetic disks are always required to have a larger recording capacity and to attain a reduction in access time such as disk rotational delay. One possible means for satisfying the latter requirement is to heighten the rotational speed of a medium.
However, the media comprising a substrate currently in use are weighed down by themselves and resonate considerably at an increased rotational speed. Eventually, the surface of such a medium comes into contact with the head to cause an error or crushing. It is therefore impossible to narrow the gap between the magnetic disk head and the recording medium to or below a certain level, and this constitutes a serious obstacle to an increase in recording capacity.
For reducing the bending of a substrate medium and diminishing the resonance of the medium being rotated, it is necessary to heighten both the modulus of elasticity (Young""s modulus) of the substrate of the medium and the rigidity thereof which is the value obtained by dividing the modulus of elasticity by the specific gravity.
The aluminum alloy which has been most commonly used as the substrates of magnetic disks has a modulus of elasticity of 71 GPa and a rigidity of 26 GPaxc2x7cm3/g. This conventional substrate material, having such properties, hardly copes with the trend toward higher rotational speeds of 10,000 rpm and above. In addition, it has become necessary to increase the thickness of substrates made of the above material, although this goes against the current trend toward thickness reduction in disk substrates for device miniaturization.
In contrast, substrates made of a tempered glass are superior to the aluminum substrate in both modulus of elasticity and specific gravity. For example, a glass substrate obtained by subjecting a commercially available soda-lime glass to ion exchange in a molten potassium salt is on the market. This substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPaxc2x7cm3/g.
Also known besides the above one is a glass substrate obtained by tempering commercially available Corning 0317. Although this substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPaxc2x7cm3/g, these properties are still insufficient.
A high-rigidity substrate for information recording media which is made of a material other than tempered glasses is on the market. This substrate comprises a crystallized glass having a modulus of elasticity of 90 GPa and a rigidity of 38 GPaxc2x7cm3/g. However, this substrate, after polishing, inevitably has residual crystal grains projecting from the surface because of the nature of the production process in which crystals are precipitated inside. Namely, this crystallized-glass substrate: has a drawback that it is inferior in surface smoothness to the substrates made of a tempered glass.
Consequently, in view of the expected future trend toward even higher rotational speeds in information recording devices and smaller thickness in disk substrates, there is a desire for a glass composition which has further improved properties, i.e., which has a high Young""s modulus and a high rigidity, can be easily tempered, and gives a substrate having high surface smoothness through polishing.
Accordingly, one object of the present invention is to provide a glass composition which has a high modulus of elasticity (Young""s modulus) and a high rigidity (modulus of elasticity/specific gravity) and is capable of being effectively inhibited from bending or vibrating when used as the substrate of an information recording medium.
Another object of the present invention is to provide a substrate for information recording media, which comprises the glass composition.
The present invention has been achieved in view of the above-described problems of prior art techniques and the above-described requirements.
The present invention provides a glass composition comprising the following components in terms of mol %: 50 to 64% SiO2, 6 to 18% Al2O3, 7 to 15% Li2O, 3 to 12% Na2O, 0 to 2% K2O, 0 to 10% TiO2, 0 to 4% ZrO2, 0 to 6% MgO, 0 to 9% CaO, and 0 to 6% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from 2 to 15%.
The glass composition preferably comprises the following components in terms of mol %: 50 to 62% SiO2, 7 to 13% Al2O3, 8 to 13% Li2O, 5 to 10% Na2O, 0 to 5% TiO2, 0 to 2% ZrO2, 2 to 5% MgO, 3 to 9% CaO, and 0 to 6% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from 5 to 15%.
The glass composition preferably has a rigidity as defined by (Young""s modulus)/(specific gravity) of 30 GPaxc2x7cm3/g or higher and a modulus of elasticity as represented by Young""s modulus of 90 GPa or higher.
Furthermore, the glass composition is preferably one which has undergone an ion exchange treatment in at least one molten salt containing ions of potassium, sodium, or both.
The present invention further provides a substrate for information recording media, which comprises the above-described glass composition which has undergone the ion exchange treatment.
This invention still further provides an information recording medium containing the substrate.
The reasons for limitations of the components of the high-rigidity high-modulus glass composition of the present invention are explained below. Hereinafter, unless otherwise indicated, all percents are by mole.
SiO2 is the main component constituting the glass. If the proportion of SiO2 is lower than 50%, the glass has impaired chemical durability. On the other hand,if the proportion thereof exceeds 64%, the desired modulus of elasticity is not obtained. Consequently, the proportion of SiO2 should be from 50 to 64%, and is preferably from 50 to 62%.
Al2O3 is an ingredient which improves the modulus of elasticity and rigidity of the glass and increases the depth of a compression stress layer formed by ion exchange. Al2O3 further serves to improve the water resistance of the glass. If the proportion of Al2O3 is lower than 6%, these effects are insufficient. On the other hand, if the proportion thereof exceeds 18%, the results are an increased viscosity, an increase in liquidus temperature which is severer than the viscosity increase, and impaired meltability. Consequently, the proportion of Al2O3 should be from 6 to 18%, and is preferably from 7 to 13%.
Li2O, which is an ingredient to be replaced in ion exchange, serves to improve the modulus of elasticity and rigidity of the glass and to lower the melting temperature of the glass to thereby enhance its meltability. If the proportion of Li2O is lower than 7%, rigidity is insufficient. On the other hand, if the proportion thereof exceeds 15%, the substrate has impaired weatherability and impaired acid resistance. Consequently, the proportion of Li2O should be from 7 to 15%, and is preferably from: 8 to 13%.
Na2O, which is an ingredient to be replaced in ion exchange, serves to lower the melting temperature and the liquidus temperature to thereby enhance meltability. If the proportion of Na2O is lower than 3%, these effects are insufficient. On the other hand, if the proportion thereof exceeds 12%, weatherability and acid resistance are impaired. Consequently, the proportion of Na2O should be from 3 to 12%, and is preferably from 5 to 10%.
K2O is an optional ingredient which enhances meltability. However, K2O not only impairs weatherability when contained in an amount exceeding 2%, but is highly effective in lowering the Young""s modulus. Consequently, the proportion of K2O is preferably not higher than 0.1%, which is an allowable limit of the amount of K2O which can come into the glass as an impurity. More preferably, the glass contains substantially no K2O.
TiO2 is an ingredient which improves the modulus of elasticity, rigidity, and weatherability of the glass. However, if the proportion thereof exceeds 10%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of TiO2 should be 10% or lower, and is preferably 5% or lower.
ZrO2 is an ingredient which improves the modulus of elasticity, rigidity, and weatherability of the glass. However, if the proportion of ZrO2 exceeds 4%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Furthermore, if the proportion thereof exceeds 2%, there is a high possibility that fine crystals might precipitate in the molten glass. Consequently, the proportion of ZrO2 should be 4% or lower, and is preferably 2% or lower.
MgO is an ingredient which heightens the modulus of elasticity, rigidity, and meltability of the glass. However, if the proportion of MgO exceeds 6%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of MgO should be 6% or lower, and is preferably from 2 to 5%.
CaO is an ingredient which heightens the modulus of elasticity, rigidity, and meltability of the glass. However, if the proportion of CaO exceeds 9%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of CaO should be 9% or lower, and is preferably from 3 to 9%.
SrO is an ingredient which heightens the modulus of elasticity and meltability of the glass. However, if the glass contains SrO in a large amount, it disadvantageously has an increased specific gravity. Consequently, the proportion of SrO should be 6% or lower.
If the total amount of MgO, CaO, and SrO (i.e., the amount of RO) is below 2%, the glass is insufficient in modulus of elasticity, rigidity, and meltability. If the total amount thereof exceeds 15%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the total amount of RO should be from 2 to 15%, and is preferably from 5 to 15% from the standpoint of high modulus.
Besides the ingredients described above, other ingredients may be added in a total amount of up to 3% for the purposes of coloring, melt clarification, etc. Examples of such optional ingredients include As2O3, Sb2O3, SO3, SnO2, Fe2O3, CoO, Cl, and F.
For molding the glass composition of the present invention, use may be made of molding techniques such as, e.g., pressing, down-draw, and float processes without particular limitations. However, a float process is optimal from the standpoints of quality and cost, because it can produce a highly flat glass plate.
This glass composition, which contains Li2O and Na2O, can be easily made to have an increased fracture strength by immersing the composition in at least one molten salt containing ions of potassium, sodium, or both at a temperature not higher than the distortion point of the glass composition to thereby interchange these ions and thus generate a compression stress on the surfaces of the composition.
When this glass composition is used as a substrate for information recording media, this substrate is less apt to bend or suffer resonant vibration because it has a higher modulus of elasticity and a higher rigidity than conventional substrates. Therefore, the recording medium employing this glass composition is especially suitable for use in recording apparatuses of the high rotational speed type.